In automatic transmissions, clutches are typically applied by the application of pressurized fluid to a clutch cylinder which causes movement of a clutch piston against a clutch pack to compress and engage the clutch pack. The pressure rise profile for the fluid used in applying the piston is typically controlled by an accumulator and spring, which results in a smooth, first order curve in the force application.
Another method of controlling the pressure rise profile is direct electronic pressure control of the clutch, wherein a pulse width controlled solenoid valve is used in connection with the accumulator and spring to control the pressure by dithering between exhaust and supply passage connections to control the pressure.
These prior art systems typically compromise between a high flow rate which provides a quick response time but poor fine control of the pressure, and a lower flow rate which results in greater fine control of pressure but lengthened response time.
Ideally, it is desirable to stroke the piston quickly while also allowing an accurately controlled, smooth pressure rise when the clutch plates are being engaged, held and released.